There is known a mobile communication system in which CSG cells (access restricted cells) settable to accept access only from particular mobile stations (a particular user group, that is, CSG: Closed Subscriber Group) can be deployed in a home, an office and the like.
In such a mobile communication system, a mobile station is capable of performing a handover from an ordinary macro cell to a CSG cell. Hereinbelow, referring to FIG. 7, a description will be given of a handover procedure which is most likely to be adopted in 3GPP.
As shown in FIG. 7, in Step S1, a mobile station UE which is in an RRC_Connected mode in a macro cell controlled by a macro radio base station Macro-eNB sends the macro radio base station Macro-eNB “Measurement Gap Request” which requests the macro radio base station Macro-eNB to assign the mobile station UE a measurement gap (measurement interval) for measuring radio qualities of CSG cells which employ different frequencies from a frequency of the macro cell.
In Step S2, the macro radio base station Macro-eNB sends the mobile station UE “Measurement Configuration” including a result of the assignment of the above-mentioned measurement gap.
In Step S3, the mobile station UE measures radio qualities of CSG cells each corresponding to “PCI (Physical Cell ID)” managed in “CSG whitelist,” in the measurement gap assigned thereto via “Measurement Configuration.” The mobile station UE then sends the macro radio base station Macro-eNB “Measurement Report (PCI Reporting)” including the radio qualities of the CSG cells.
In this respect, a PCI is repeatedly used in the mobile communication system. As to CSG cells, in particular, it is assumed that there are multiple CSG cells employing a common PCI, in a coverage of a single macro cell.
As described above, there is a case where a single PCI is associated with multiple CSG cells. Thus, in the case where the PCI included in “Measurement Report (PCI Reporting)” thus received cannot uniquely specify a single CSG cell, the macro radio base station Macro-eNB sends in Step S4 the mobile station UE “GCI Report Request” which requests the mobile station UE to transmit “Measurement Report (GCI Reporting)” on a CSG cell specified by “GCI (Global Cell ID).”
In Step S5, in response to “GCI Report Request” thus received, the mobile station UE sends the macro radio base station Macro-eNB “Measurement Report (GCI Reporting)” on the CSG cell specified by the GCI. The Measurement Report includes at least a GCI or a CSG-ID.
Here, Steps S2 and S4 may be performed concurrently, and Steps S3 and S5 may be performed concurrently.
In Step S6, the macro radio base station Macro-eNB determines that the mobile station UE has to perform a handover from the macro cell controlled by the macro radio base station Macro-eNB itself to a CSG cell controlled by a home radio base station Home-eNB. The macro radio base station Macro-eNB thus sends the home radio base station Home-eNB “HO Preparation Request” requesting the home radio base station Home-eNB to prepare for the handover.
In Step 7, when having successfully reserved a resource for the handover, the home radio base station Home-eNB sends the macro radio base station Macro-eNB “HO Preparation ACK.”
In Step 8, when having received “HO Preparation ACK,” the macro radio base station Macro-eNB sends the mobile station UE “HO command” to instruct the mobile station UE to perform the handover.
In Step 9, after having completed processing related to the handover to the home radio base station Home-eNB, the mobile station UE sends the home radio base station Home-eNB “HO Complete.”
The handover procedure described above, which is most likely to be adopted in 3GPP, however, has a problem of lack of a specific definition for how the macro radio base station Macro-eNB should cause the mobile station UE to measure a radio quality of a CSG cell, by use of “Measurement Configuration”, and a specific definition for how the mobile station UE should report the measurement result of the radio quality of the CSG cell by use of “Measurement Report.”